For that you do not even need a nuclear device. It can be driven by conventional explosives (I will not go into the details of what else is involved, except for no fissile material), and on the surface it appears to have been a car bomb attack (just think of what if the Times Square car bomb worked).
The jihadis are not all just a bunch of stupid goathumpers. There are some among them with engineering degrees - just look at the educational background of the 19 hijackers of September 11.
Yes I am aware of the operating principles of the device described. I simply did not wish disseminate any of the information to those that may attempt to use it.